1. Field of the Invention
The present invention relates generally to a multiple antenna system. More particularly, the present invention relates to a method and an apparatus for calibration in a multiple antenna system that supports a Relay Station (RS).
2. Description of the Related Art
System performance and capacity of mobile communication systems are limited by interference signals of the same channel between cells or within a cell, and radio channel characteristics such as multipath fading and the Doppler effect. To address the performance degradation factors, one technique for expanding the system capacity is a beamforming technique.
The beamforming technique directs a radio signal toward each terminal by multiplying downlink traffic transmitted to each terminal by a particular coefficient. To apply the beamforming technique to a Base Station (BS), it is necessary to calibrate a phase and amplitude difference caused by the characteristic difference of Radio Frequency (RF) elements of the downlink and the uplink of the BS.
A conventional calibration method relatively calibrates the phase and amplitude difference of the uplink per path. Likewise, the conventional calibration method relatively calibrates the phase and amplitude difference of the downlink per path. That is, the phase and the amplitude are calibrated equally per downlink and uplink based on Equation (1). Herein, four transmit antennas and four receive antennas are under consideration.Hd1*Wd1=Hd2*Wd2=Hd3*Wd3=Hd4*Wd4 Hu1*Wu1=Hu2*Wu2=Hu3*Wu3=Hu4*Wu4   (1)
In Equation (1), the channel of the downlink path 1 is Hd1, the channel of the uplink path 1 is Hu1, the channel of the downlink path 2 is Hd2, the channel of the uplink path 2 is Hu2, the channel of the downlink path 3 is Hd3, the channel of the uplink path 3 is Hu3 the channel of the downlink path 4 is Hd4, and the channel of the uplink path 4 is Hu4. Wdx denotes a beamforming calibration coefficient for the downlink path x and Wux denotes a calibration coefficient for the uplink path x.
The conventional method determines the calibration coefficients Wd1, Wd2, Wd3, Wd4, Wu1, Wu2, Wu3 and Wu4 to establish Equation (1). Moreover, the conventional method establishes the relationship of Hd1/Hu1=Hd2/Hu2=Hd3/Hu3=Hd4/Hu4=α (an unknown complex value including both the phase and the amplitude).
When the same unknown phase and amplitude difference α between the downlink and the uplink exists in each path, the beamforming coefficient is expressed by Equation (2).
                                          w            t                    =                                    α              ⁢                                                          ⁢                              h                t                *                                                                                                                                                    h                      1                                                                            2                                +                                                                                                h                      2                                                                            2                                +                                                                                                h                      3                                                                            2                                +                                                                                                h                      4                                                                            2                                                                    ,                  t          =          1                ,        2        ,        3        ,        4                            (        2        )            
In Equation (2), wt denotes the t-th beamforming coefficient and ht denotes the t-th channel matrix, ht* denotes the conjugate of ht.
The receive signal beamformed based on Equation (2) is given by Equation (3).r=α√{square root over (|h1|2+|h2|2+|h3|2+|h4|2)}·s+n   (3)
In Equation (3), α denotes a phase difference, s denotes a transmit vector, ht denotes the t-th channel matrix, and n denotes a noise vector.
To maximize a Signal to Noise Ratio (SNR) of the receive signal, a receiver of the terminal calibrates the phase of the receive signal. Accordingly, the value corresponding to the phase difference disappears from the value α, and merely the amplitude difference remains. In an ideal case, the SNR difference corresponds to the square of the amplitude difference and does not affect the beamforming performance.
The above-mentioned calibration method does not incur any problem in a BS that does not use a Relay Station (RS), but causes a problem in a system supporting an RS. The problem that occurs in a conventional system supporting an RS is now explained by referring to FIGS. 1A and 1B.
In FIG. 1A, when a Mobile Station (MS) 130 is far away from a BS 100, that is, when an RS 110 is propagationally isolated from the BS 100, such as being underground, on an “island”, or in a shadow area, the beams 150 and 155 are formed in accordance with the sounding channel although the sounding is incoming via the RS 110. The beam corresponding to the interval between the RS 110 and the MS 130 is formed to the RS 110 and the BS 100 at the same time. However, since one radio resource is allocated to only one MS 130 in the region covering the BS 100 and the RS 110, there is no problem in the beamforming.
In FIG. 1B, when the MS 130, which is located between the BS 100 and the RS 110, simultaneously transmits sounding signals to the BS 100 and the RS 110, both of the uplink sounding signals 135 and 140 transmitted from the MS 130 are received at the BS 100 and the RS 110 and their combined signal is input to the BS 100. When a radio channel from the MS 130 to the BS 100 is h and a radio channel from the MS 130 to the RS 110 is h′, the beamforming coefficient may be determined at the BS 100 using the current algorithm based on Equation (4).
                              w          t                =                                            (                                                α                  ⁢                                                                          ⁢                                      h                    t                                                  +                                  βh                  t                  ′                                            )                        *                                                                                                                                            h                      1                                        +                                          h                      1                      ′                                                                                        2                            +                                                                                                            h                      2                                        +                                          h                      2                      ′                                                                                        2                            +                                                                                                            h                      3                                        +                                          h                      3                      ′                                                                                        2                            +                                                                                                            h                      4                                        +                                          h                      4                      ′                                                                                        2                                                                        (        4        )            
The signal received at the MS 130 is given by Equation (5).
                    r        =                                                            ∑                                                                  ⁢                                  {                                                                                                                                                                    h                            t                                                                                                    2                                            ⁢                                              α                        *                                                              +                                                                                                                                                  h                            t                            ′                                                                                                    2                                            ⁢                                              β                        *                                                              +                                                                  h                        t                        *                                            ⁢                                              h                        t                        ′                                            ⁢                                              α                        *                                                              +                                                                  h                        t                                            ⁢                                              h                        t                                                  ′                          *                                                                    ⁢                                              β                        *                                                                              }                                                                                                                                                                                                h                          1                                                +                                                  h                          1                          ′                                                                                                            2                                    +                                                                                                                                    h                          2                                                +                                                  h                          2                          ′                                                                                                            2                                    +                                                                                                                                    h                          3                                                +                                                  h                          3                          ′                                                                                                            2                                    +                                                                                                                                    h                          4                                                +                                                  h                          4                          ′                                                                                                            2                                                                        ·            s                    +          n                                    (        5        )            
In Equation (5), α denotes the phase and amplitude difference of the downlink and the uplink at the BS, β denotes the phase and amplitude difference of the downlink and the uplink at the RS, ht denotes the t-th channel matrix, s denotes a transmit vector, and n denotes a noise vector.
In Equation (4) and Equation (5), when the MS between the RS and the BS transmits the sounding signals, the SNR differs depending on α and β. In other words, the beamforming performance is influenced by α and β.
As discussed above, since the phase and amplitude difference α of the downlink and the uplink of the BS 100 may differ from the phase and amplitude difference β of the downlink and the uplink of the RS 110, the multiple antenna system including the RS is subject to beamforming performance degradation.